The United States and Japan will step up their defence cooperation to deal with the threat from nuclear-armed North Korea as tensions in East Asia remain high, officials from the two allies said on Thursday.

Flash flooding forecast in regional Vic

Flash floods are possible across much of regional Victoria on Thursday.

The Bureau of Meteorology has issued a severe weather warning for the Mallee, Wimmera, Northern Country, North Central, North East and South West regions as storms associated with a low pressure trough cross the state.

Storms from the same weather system hit hard across parts of South Australia, cutting power to hundreds of homes, causing flash flooding and forcing the State Emergency Service to respond to 70 calls for assistance.